| Yep, there's a lot to
think about when converting an old carbureted car into a an EFI. An EFI
system operates at a higher fuel pressure, typically around 45 psi (3
bar), and the pressure needs to be steady, yet change linearly in
reference to manifold pressure. In other words, the pressure
differential across the fuel injector must remain constant. Late model
cars use an an in-tank fuel pump submerged in fuel, and may have an
integral pressure regulator. That way there's no need for a fuel return
line, but it doesn't take into account the pressure losses across the
fuel lines and filter. Some OEM's have a pressure sensor at the
fuel rail, and the ECU controls the voltage to the fuel pump to adjust
pressure. That said, your best bet is to mount a by-pass regulator in
the engine compartment, and route a fuel return line (by-pass) back to
the tank. At idle, the regulator sends most of the fuel volume back to
the tank, so you must be careful not to create any restrictions in the
return line - otherwise the regulator cannot do it's job. The
return line should be at least the same size as the pressure line. |
 At first I was tempted to mount the fuel return line into
the gas filler tube, as it would have been ten times less work to
just remove the fill tube from the tank instead of dropping the whole
tank. But then I decided to do it properly.I fabricated the return line to the tank out of 10 mm (3/8") stainless tubing. Inside the tank, it extends all the way to the bottom. I recall reading somewhere that the return line must be routed under the fuel level inside the tank, as otherwise there's a risk of aerating the fuel. The real challenge was to form the portion of tube inside the tank to get around the fuel sender, yet allow installation of the complete assembly through that small aperture! While at it, I checked the fuel level sender to be working correctly. This is not Buick's original tank, but another GM tank that followed with the project car. The sender reads 2 ohms empty to 38 ohms full. The GM spec has been a nominal 0 to 30 ohms for decades, at least to the 70's. The sender has a cork float so it's pretty old. |
 Optimally, the pump should be mounted below the fuel level, in order for the fuel to "siphon" from the tank. This prevents cavitation inside the pump, that could vaporize the fuel and eventually damage the pump. This is the best mounting spot I could come up with - inside the rear frame rail. It's not below tank but not above either. The fuel pump is an MSD #2225 which I purchased around 2003 when I started disigning a Megasquirt EFI. It's good for approximately 500 HP. Capable of 43 gph @ 40 psi or 39 gph @ 80 psi, takes 5 amps. The filter is a Bosch unit that I found at a parts counter. I don't have any kind of fuel filter on the suction side, except the usual "sock" inside the tank. A filter on the suction side might cause - you quessed it - cavitation. The thing on the lower front is a muffler. The tank, once installed, is going to block this view. Of course, the pump needs a power supply from the battery, through a relay controlled by the ECU, so the pump stops when the engine stops. |
 On the left upper corner of the photo, you will see the Aeromotive #13105 fuel pressure regulator, mounted on the firewall above the horn. As for the fuel return line, I made it out of 3/8" stainless-braided teflon tube. I know it costs a fortune, but I'm too old to crawl under the car for extended periods, so I refused to fabricate a hard line. I should have had the foresight to install a return line when the project was in a frame-off state - but I didn't. OOOPPS! I didn't intend to reveal the coil packs yet! More of that later. |
| To be continued ;-) |